1. Field of the Invention
The present invention relates to sealing a temperature transmitter to prevent leakage of fluids through a conduit connected to control equipment.
2. Description of the Prior Art
Temperature sensors for process fluids are typically fitted with a primary seal, such as a thermowell 10 in a PRIOR ART installation shown in FIG. 1. The primary seal blocks leakage or flow of process fluid 12 past a sensor 14 into an electrical conduit 16 carrying the temperature sensor's electrical leads 18 to a converter 19 in a compartment 20 of temperature transmitter 11. Another electrical conduit 22 carries output leads 24 from converter 19 to a remote location such as a control room. In the event of failure or leakage of the primary seal, pressurized process fluid can flow past the sensor through conduit 16, then through compartment 20, and finally through conduit 22 to the control room. In addition to the problem of primary seal failure, there is also a problem in some installations with condensation of moisture or corrosive fluids from the air which fills the conduit connecting the temperature sensor with the temperature transmitter. Gasses can enter the conduit through leaks and vents, as well. Process fluid, condensation or gasses leaking into control rooms can damage equipment or introduce noxious gasses or ignition problems into the control room.
To block flow of fluids to control equipment, provision of a seal in the conduit system between the transmitter and the control room is desired, and in some instances is required by the National Electrical Code (NEC), paragraph NEC 501-5.f.3. In the past, conduits has been fitted with a sealing device such as drain-seal 26 shown in FIG. After leads 24 have been pulled through the drain-seal, the drain-seal is filled with a packing or potting compound 27 to block leakage of fluid. Additional drain-seals have also been installed between the thermowell and the transmitter. Installation of the drain-seal requires several installation steps which can be inadvertently overlooked during field installation. Testing the secondary seal under field conditions is difficult, and the installation of the drain-seal is costly and time consuming.
Another PRIOR ART temperature transmitter shown in FIG. 2 is similar to the transmitter shown in U.S. Pat. No. 4,623,266 to Kielb, incorporated herein by reference, and has converter circuitry 30 installed in a compartment 32 which is sealed from a wiring compartment 34 containing terminals for leads for both the temperature sensor and the transmitter output. The converter circuitry 30 is sealed from the process fluid, but fluid can still flow between conduit entry ports 36 through the wiring compartment 34 to a remote location, necessitating a separate drain-seal with this transmitter as well.
A different arrangement is desired which avoids the use of such a separate drain-seal in electrical conduit systems connecting temperature transmitters to control equipment while still maintaining a seal which blocks leakage of fluids from the transmitter to the remote location